The present invention relates generally to a radio-frequency identification (RFID) antenna that is designed to operate in proximity to metal surfaces. Specifically, the antenna is positioned at 90 degrees to the surface, allowing the antenna to operate with minimal separation from the edge of the RFID antenna to the metallic object. The present subject matter is especially suitable for food and medication containers. In accordance with embodiments of the present subject matter, an RFID antenna is provided that is designed to operate in proximity to conductive surfaces, with the surfaces including high dielectric constant and high dielectric loss, such as some liquids, gels, solutions, and combinations of these surfaces and material. Particular relevance is found in connection with sealed food and medication containers. Accordingly, the present specification makes specific reference thereto. However, it is to be appreciated that aspects of the present inventive subject matter are also equally amenable to other like applications.
Radio-frequency identification (“RFID”) is the use of electromagnetic energy (“EM energy”) to stimulate a responsive device (known as an RFID “tag” or transponder) to identify itself and in some cases, provide additionally stored data. RFID tags typically include a semiconductor device commonly called the “chip” on which are formed a memory and operating circuitry, which is connected to an antenna. Typically, RFID tags act as transponders, providing information stored in the chip memory in response to a radio frequency (“RF”) interrogation signal received from a reader, also referred to as an interrogator. In the case of passive RFID devices, the energy of the interrogation signal also provides the necessary energy to operate the RFID device.
RFID tags may be incorporated into or attached to articles to be tracked. In some cases, the tag may be attached to the outside of an article with adhesive, tape, or other means and in other cases, the tag may be inserted within the article, such as being included in the packaging, located within the container of the article, or sewn into a garment. The RFID tags are manufactured with a unique identification number which is typically a simple serial number of a few bytes with a check digit attached. This identification number is incorporated into the tag during manufacture. The user typically cannot alter this serial/identification number and manufacturers guarantee that each serial number is used only once. This configuration represents the low cost end of the technology in that the RFID tag is read-only and it responds to an interrogation signal only with its identification number. Typically, the tag continuously responds with its identification number. Data transmission to the tag is not possible. These tags are very low cost and are produced in enormous quantities.
Such read-only RFID tags typically are permanently attached to an article to be tracked and, once attached, the serial number of the tag is associated with its host article in a computer data base. The RFID tag data, both a unique ID and data stored in a read/write memory, may also be associated in a database with a host article, but not always. The tag may store data read from a bar code, or the item identification, its manufacturing date etc. and have no association with a database or requirement to access one.
Read only tags, those that respond with a pre-programmed code when powered up at a regular or pseudo random interval, are no longer commonly used.
Most tags now incorporate chips that include both read only memory, that usually contains configuration bits, manufacturers ID, chip model number and a unique ID ranging between 2 and 9 bytes in length, and read write memory commonly between 12 and 16 bytes, although larger memories may be used. The unique ID is used in combination with the manufacturers ID and chip model number (two different chip manufacturers could use the same unique ID).
Specifically, an object of the tag is to associate it with an article throughout the article's life (the tag may be applied at any point in the supply chain, not necessarily for the articles life) in a particular facility, such as a manufacturing facility, a transport vehicle, a health care facility, a pharmacy storage area, or other environment, so that the article may be located, identified, and tracked, as it is moved. Tracking the articles through the facility can assist in generating more efficient dispensing and inventory control systems as well as improving work flow in a facility. This results in better inventory control and lowered costs. In the case of medical supplies and devices, it is desirable to develop accurate tracking, inventory control systems, and dispensing systems so that RFID tagged devices and articles may be located quickly should the need arise, and may be identified for other purposes, such as expiration dates or recalls.
Many RFID tags used today are passive in that they do not have a battery or other autonomous power supply and instead, must rely on the interrogating energy provided by an RFID reader to provide power to activate the tag. Passive RFID tags require an electromagnetic field of energy of a certain frequency range and certain minimum intensity in order to achieve activation of the tag and transmission of its stored data. Another choice is an active RFID tag; however, such tags require an accompanying battery to provide power to activate the tag, thus increasing the expense and the size of the tag and making them undesirable for use in a large number of applications.
Depending on the requirements of the RFID tag application, such as the physical size of the articles to be identified, their location, and the ability to reach them easily, tags may need to be read from a short distance or a long distance by an RFID reader. Furthermore, the read range (i.e., the range of the interrogation and/or response signals) of RFID tags is also limited.
Furthermore, when the RFID tags are attached to a conductive surface, an RFID tag may have difficulties in being read. In those situations where reading a tag is problematic, such as where the space between a dipole and its image is small reducing the space creates difficulty in reading the tag, such as where the space is very small (less than one wavelength), then the total effective current between the dipole and its image is equal to zero or near zero. As the spacing between the antenna and the metal plane decreases the efficiency of the antenna reduces and it becomes difficult to achieve an impedance match to a device such as an RFID chip over a useful bandwidth. The issues become more apparent when the spacing is ˜<1% of one wavelength; these problems can be mitigated to some extent by using a separator between the antenna and plane. For example, a high dielectric constant material may be used, or a material with both a high dielectric constant and high relative permeability, to increase the effective separation. However, such materials are expensive, and not suitable for RFID tags, where lower cost materials, such as papers/card, simple plastics such as PET and polypropylene foams are more desirable; all of these have relatively low dielectric constants.
Thus, the total radiated field is negligible and therefore, the RFID tag is unable to capture data and power from the reader. This is a significant problem given that in many commercial applications it is desirable to apply the RFID tag to a metal or other type of conductive surface. What is needed therefore is an RFID tag device and/or system that allows the RFID tag to operate in proximity to metal surfaces or other types of conductive surfaces.
The present invention discloses an RFID antenna structure that is designed to operate in proximity to metal surfaces. The RFID antenna structure is placed at 90 degrees to the surface of the metallic object, allowing it to operate with minimal separation from the edge of the RFID antenna structure to the metallic object.